Multilayer compositions have been useful for many years. The concept of a multilayer is that the positive properties of two or more materials are combined with the structural integrity of each material being essentially uncompromised. Usually the positive properties of one material offset or counterbalance the weaker properties of the second material. For example among the positive properties of polycarbonate are high heat resistance and impact strength. However, polycarbonate has a relatively high transmission rate for certain gases, oxygen and carbon dioxide, for example. Ethylene vinyl alcohol copolymer does not have very high resistance to impact or heat but has excellent resistance to the transmission of various vapors and fluids such as oxygen and carbon dioxide. Therefore a multilayer composition utilizing polycarbonate adjacent to ethylene vinyl alcohol can be employed in structures wherein the properties of high impact resistance, high heat resistance and high resistance to oxygen and carbon dioxide transmission are necessary.
Although many of these multilayer compositions can be hypothesized on the basis of laminating a material possessing certain strong properties with a material having weaknesses in these same property areas, certain practical considerations inhibit successful implementation on this theory. The two materials are in intimate contact at their interface. This juncture of interface should provide a sufficiently strong interaction with respect to the processing conditions which the multilayer structure undergoes that a tight, firm bond is maintained. Such conditions to which the multilayer can be exposed include heat, pressure, humidity, liquid chemicals, gases, and the like or various combinations of these conditions. The propensity of the two layers to maintain this tight, firm bond is generally known as the "compatibility" of the two layers. When the two materials are incompatible the utility of the multilayer structure is severely inhibited or useless. In order to bind two relatively incompatible layers, a tie layer is generally employed joining the two incompatible layers by "tieing" them together. This tie layer usually enjoys good compatibility with both incompatible layers and is a layer between the two incompatible layers.
The aforementioned multilayer combination of polycarbonate and ethylene vinyl alcohol suffers from this incompatibility. In fact, ethylene vinyl alcohol is compatible in a multilayer situation with very few other thermoplastics. Its peel strength, that is the amount of force needed to peel apart the ethylene vinyl alcohol layer from the layer in which it is intimate contact is extremely low, thereby providing an extremely weak multilayer composite.
Various solutions have been directed to effectively combining ethylene vinyl alcohol polymers with other thermoplastic layers. Specifically, U.S. Pat. No. 4,332,858 issued to Saitoh, et al and assigned to Asahi Kasei Kogyo Kabushik Kaisha specifically deals with the problem of compatibilizing ethylene vinyl alcohol polymers with other thermoplastic polymers so as to provide strong interlaminar adhesion, i.e. peeling strength. The solution which is disclosed in the Saitoh '858 patent is the use of a specific tie layer prepared from a block copolymer of a polymerized alkyldiene and a vinyl aromatic. Block copolymers of butadiene and styrene are disclosed and employed. The interlaminar adhesion of the system is substantially increased by the use of a graft modified butadienestyrene block copolymer. The graft modifying agent is an unsaturated dicarboxylic acid or dicarboxylic acid anhydride. These agents include such materials as maleic acid, fumaric acid, itaconic acid, and the anhydrides thereof, specifically maleic anhydride. However, even with the grafting agent on the random block copolymer, only polybutadiene can be employed as the alkyldiene substituent. According to Saitoh, isoprene cannot be employed.
However, the Saitoh invention does not provide sufficient thermal stability for the use of that tie material with high temperature thermoplastic materials in multilayer laminates with ethylene vinyl alcohol in a commercial fashion, for example by coextrusion.
It has now been discovered that a further modification of the styrene-butadiene type block copolymer together with the grafting agent of the unsaturated dicarboxylic acid or dicarboxylic acid anhydride will bring about a system which can undergo successful commercial exploitation. Additionally the butadiene unit present in the block copolymer can now be expanded to include isoprene or virtually any alkyldiene unit which can be copolymerized with a vinyl aromatic.